a) Field of the invention
This invention relates to a brush seal assembly suitable for effecting a seal between a housing and a shaft rotatable with respect to that housing.
b) Description of the Prior Art
In many machines, it is necessary for a seal to be effected between a housing and a shaft mounted for rotation with respect to the housing, for example to prevent the egress of lubricants out of the housing, or the ingress of foreign matter to bearings supporting the shaft. There have been innumerable designs for seals able to fulfill such functions, and the selection of an appropriate type of seal for a given set of operating conditions and circumstances does not normally present any great difficulty. However, a special problem arises in the case of a shaft which is intended to rotate at relatively high speeds, such as is encountered in gas turbines, jet engines and similar machines, to effect fluid (i.e. gas and/or liquid) seals either at the points where the shaft leaves the housing or between separate compartments within the machine. Such shafts may rotate at speeds as high as 100,000 rpm, and most conventional designs of seal are not suitable for use in these situations.
The problems of sealing shafts which are intended to rotate at very high rotational rates has led to the development of so-called brush seals, where an annular carrier is provided with bristles which, in the completed seal where the carrier is mounted co-axial with the shaft, have their tips wiping against the circumferential surface of the shaft. The bristles of a brush seal typically are made from a metallic wire and the seal has to be manufactured to very close radial tolerances, in order that an effective seal may be obtained without also causing heavy wear on the shaft, as a consequence of the tips of the bristles wiping the shaft surface. Moreover, for the same reasons the seal must be finished to a high order of circularity and must be mounted precisely concentrically with the shaft.
A brush seal equally may be designed for mounting on the shaft itself, with radially outwardly directed bristles to engage a suitable bore in the housing through which the shaft passes, but such seals are less commonly used in view o f the difficulties of satisfactorily mounting the seal on the shaft. Other possibilities include having the bristles extending parallel to the shaft axis, with the tips of the bristles sealing against a radial surface defined either by a shoulder on the shaft in the case of a seal mounted on a housing, or against an end face of the housing in the case of a seal mounted on the shaft.
As mentioned above, the known forms of brush seal are manufactured by providing an annular carrier on which are mounted the seal bristles, the seal then being machined to very close tolerances. When the seal is to be employed, it must be suitably mounted on the housing so as to surround the shaft and effect a seal thereagainst (for the usual seal configuration as described above) but this can be done only if there is access to the housing axially of the shaft, at the time the seal is to be fitted. The construction of certain engines therefore requires the seal to be fitted in place during assembly of the main components, rather than at the end of the assembly procedure. Moreover, should it be necessary to change the seal during the life of the machine, such axial access may necessitate most considerable dismantling of the machine. These difficulties may make the initial assembly of the machine - such as a large gas turbine or jet engine for an aircraft - difficult, especially if the shaft has to be assembled after the seal has been mounted on the housing which action itself may damage the seal. Also, if the seal is to be changed when the machine is in service, these difficulties may give rise to undesirably long service times.